1. Field
The present invention relates to optical transmission systems, and more particularly, to an optical transmission system for compensating for waveform distortion of optical signals.
2. Description of the Related Art
In optical communication systems using optical fibers as transmission paths, waveform distortion occurs due to chromatic dispersion, and therefore, design of chromatic dispersion is of great importance. If the amount of chromatic dispersion is too large, optical signal waveforms are distorted under the influence of spectral spread attributable to SPM (Self Phase Modulation), degrading the signal quality. If the dispersion is too small, on the other hand, significant interference of waveforms occurs during the wavelength division multiplex transmission due to crosstalk attributable to FWM (Four Wave Mixing) and also due to XPM (Cross Phase Modulation) of adjacent channels, leading to degradation of the signal quality. Accordingly, transmission paths are usually made to have a dispersion of about 2 ps/nm/km so that when the dispersion accumulates to about several hundred ps/nm, the cumulative dispersion may be compensated.
A maximum number of WDM (Wavelength Division Multiplexing) channels is restricted by the OSNR (Optical Signal to Noise Ratio), which is determined by the characteristics of optical repeaters and the loss of optical fibers, and also by signal degradation caused within the transmission paths. Signal degradation is caused typically by fiber nonlinear SPM and GVD (Group Velocity Dispersion) (SPM-GVD). In designing novel systems, it is necessary that degradation attributable to SPM-GVD be lessened in order to increase the number of channels. Also, in the case of upgrading existing systems that are already providing services, degradation attributable to SPM-GVD needs to be reduced to maximize the number of available channels. The state of such nonlinear degradation changes because optical power per channel varies with increase/decrease in the number of channels in service.
As techniques for compensating for the chromatic dispersion of optical fibers, there have been known, besides optical compensation methods, electronic compensation methods for electrically compensating for distortion of optical signal waveforms. Electronic compensation methods include: 1. method in which signal waveforms are directly detected and bandwidth equalization is performed on the waveforms of the received electrical signal; and 2. method in which waveform distortion caused within a transmission path is compensated through amplitude modulation and phase modulation. The latter method utilizes information on the phase of optical signal and therefore can compensate for dispersion with an accuracy about ten times as high as that achieved by the former method.
As an example of the latter method, a technique is known in which compensating modulation is carried out at the signal transmitting device (transmitting station) (see, e.g., PCT-based Unexamined Japanese Patent Publication No. 2006-522508 filed by Nortel Networks Limited, and “Electronic Dispersion Compensation Tourniquets for Optical Communications Systems”, ECOC 2006 Tu3.2.1). For example, assuming that the transmit signal is E(t), the waveform C[E(t)] of the received signal can be obtained by transmission simulation. The transmission simulation can be implemented, for example, by solving the nonlinear Schrodinger equation by the split-step Fourier method (see, e.g., “Nonlinear Fiber Optics” by Agrawal, 2nd edition, p. 45). C[] can be obtained from parameters including the transmit signal waveform, the optical power (transmit optical power, optical power on the transmission path), the amount of dispersion, the nonlinear optical constant of the optical fiber, and the system length. Also, C[] is a complex function and carries information on both amplitude and phase.
If C[E(t)] can be obtained as the received signal, then it is possible to derive C−1[]. The transmitting station applies C−1[] to the transmit signal E(t) to subject the signal to the waveform conversion C−1[E(t)] and transmits the resulting signal, and since C[C−1[E(t)]]=E(t), the receiving device (receiving station) can derive the original signal waveform without the need for dispersion compensation.
A technique is also known in which compensating modulation is performed at the receiving station (see, e.g., “1.6 Gbit/s Real-Time Synchronous QPSK Transmission with Standard DFB Lasers” by the University of Paderborn, ECOC 2006 Mo4.2.6). Provided that the transmit signal is E(t), for example, the waveform C[E(t)] of the received signal can be obtained in advance by the transmission simulation. Thus, by applying C−1[] to the received signal at the receiving station, it is possible to obtain the original signal waveform because C−1[C[E(t)]]=E(t).
Where distortion is compensated only at one of the transmitting and receiving stations, however, the amount of compensation occasionally becomes large, giving rise to a problem that the waveform distortion cannot be significantly reduced.